Spruce seems to have been a generic term for commodities brought to England by Hanseatic merchants (especially beer, boards and wooden chests, and leather), and the tree thus was believed to be particular to Prussia, which for a time was figurative in England as a land of luxuries. Compare spruce (adj.).
As a distinct species of evergreen tree from 1731. Nearly all pines have long, soft needles growing in groups of two (Scotch) to five (white); spruce and fir needles grow singly. Spruce needles are squarish and sharp; fir needles are short and flat. Cones of the fir stand upright; cones of a spruce hang before falling.
1550s (n.), "native or inhabitant of Prussia;" 1560s (adj.), "of or pertaining to Prussia;" from Prussia + -an. In reference to the language of the earlier inhabitants of (East) Prussia, which was closely related to Lithuanian, by 1888. It was spoken between the lower Vistula and the Niemen and was extinct by the end of 17c. Prussian blue pigment (1724) came to English from French bleu de Prusse, so called for being discovered in Berlin, the Prussian capital.
All in all, it seems that Prussian blue was synthesised for the first time around 1706 by the Swiss immigrant Johann Jacob Diesbach in Berlin. [Jens Bartoll and Bärbel Jackisch, "Prussian Blue: A Chronology of the Early Years," in Zeitschrift für Kunsttechnologie und Konservierung 24, No. 1, 2010]
Early German sources refer to it as Preußisches Ultra-Marin and berliner blau. Prussic acid (1790), is from French acide prussique, so called in reference to Prussian blue pigment, to which it is chemically related.
type of small Spitz-type dog, 1760, from Pomerania, former province of Prussia on the south coast of the Baltic Sea.
also psycho-graphic, "of or pertaining to psychography," 1856, from psychograph "supernatural photographic image or device" (1854) from psycho- + -graph. Also see psychography. Related: Psychographics.
—What next? Among the new patents announced is one to Adolphus Theodore Wagner, of Berlin, in the kingdom of Prussia, professor of music, for the invention of a "psychograph, or apparatus for indicating a person's thoughts by the agency of nervous electricity." [Arthur's Home Magazine, May 1854]